This invention relates to ultrasonic flaw detectors for detecting flaws in material, structural assemblies, etc., and, more particularly, to an ultrasonic flaw detector implementing DGS curves and a method for implementing and using curves in such a detector.
Ultrasonic flaw detection is used in many manufacturing applications to detect imperfections within structural elements. As is well-known in the art, the detection method involves generation a sound wave or pulse, transmitting the pulse through a transducer or probe into the material, listening for a sound return, an echo or "ping", and then evaluating the characteristics of the echo. The amplitude of the echo is a function of a number of factors; these include, the frequency of the sound wave, the sound transmission characteristics of the material being tested, the size of the flaw, and the transit time of the pulse from its point of injection into the material to the discontinuity and the return time. Recently, flaw detector instrumentation has been developed which not only electronically processes the returns from a flaw detection test, but also provides both digital results of the test together with a graphic display of a return. This allows a user to readily view an amplitude versus distance display of the test result. For determining whether or not the material under test passes a particular test, the display may include some type of reference indication which visually represents the maximum allowable echo amplitude which is acceptable. If the displayed amplitude exceeds this "go/no-go" indication, the material or assembly fails the test. The acceptance level is also based upon a number of factors related to the material or structure, etc.
It will be appreciated that with a point source of sound transmission such as is provided by a transducer, the transmitted pulse will disperse in a geometric manner. This, together with the normal attenuation resulting from passage of the pulse through the material requires that the amplitude of the processed echo be corrected based on these considerations. Otherwise, for example, the displayed amplitude of the processed echo may be such that a part which is actually faulty could be accepted, because the flaw may be sufficiently deep into the material that the processed, attenuated amplitude of the echo for that depth falls within the acceptable range of values.
This problem has been recognized and various approaches have been taken to attempt to adjust the measured values for these factors so the adjusted result is an accurate indication of the test result. One approach which has been used involves distance gain sizing (DGS) and uses a DGS curve. DGS is also referred to as AVG in Europe. In use, a DGS curve is actually a series of curves, separate ones of which are created for not only each of the types of probes or transducers which are used, but also for the range or depth of material which is being tested, the overall system gain of the testing device, and similar related parameters. There may be 10-15 curves for each type of transducer used. The curves provide a compensation factor to adjust the measured amplitude of an echo so the corrected amplitude can be compared against the reference to determine whether a test is passed or failed. DGS curves have been in use for some time. However, while they provide a high degree of correctabilty, the need for a wide variety of charts, and the limitations the charts impose, has made this approach cumbersome to use. For example, if a baseline is made for calibration purposes using a particular transducer and range of reflections, all subsequent tests had to be made within the same ranges or else a new baseline established each time the range is changed. The recently developed electronic instrumentation does not use DGS because of the various drawbacks outlined above.
DGS is a methodology which was developed in Germany in 1958. During this period, a separate compensation methodology known as distance-amplitude correction, or DAC, was developed in the United States. This DAC procedure involves measuring the response from a known reflector (a 0.064-inch diameter disk, for example) at different distances from a source. The responses are then used to develop correction factors so that the amplitude of the response is constant for the range of measured distances. It will be understood that either approach is a valid way by which test data can be interpreted to determine the acceptability of material under test.
Although electronic based flaw detectors are currently in use which employ DAC as part of the processing methodology, there is still a desire to use DGS curves with such instruments. For the digital instrumentation now available, people have prepared transparent overlays of the curves which fit over the display screen. Others use a grease pencil or similar implement to draw an appropriate DGS curve on the faceplate over the display. Not only are these approaches impractical, the user still must have available a large number of DGS curves for use under a wide range of test conditions. Or else, the instrument can only be configured for use with one type transducer, for example, and during the testing there can be no change in the transducer, the instrument's gain setting, testing range, offset or delay, etc.